This invention is directed to constructions for the handling of powder and, in particular, to constructions which serve to deliver powder in a carefully controlled fashion. The invention is particularly usable in operations which involve the application of powder to the printed surfaces of sheets of paper or plastic and the like.
Electrostatic powder sprayers are widely employed as a means for applying anti-offset powders to the printed surfaces of sheets, webs and the like. Such application of powders takes place after printed material emerges from a printing press.
Powder utilized in such operations typically comprises a random mixture ranging in size from 5 to 100 microns. Because of the size of the particles involved, numerous problems have developed when attempting to uniformly distribute the powder over a sufficiently long period of time to provide an efficient operation. The small particle size, coupled with the size range involved, results in a material which cannot be handled on a predictable basis with systems previously available for use.
One attempt to solve the handling problems involved the use of a carburetor which fluidized powder in a jar-like container with the powder being contained in a rapidly moving air stream for movement to distributing means. Such systems utilize a needle valve or the like for increasing or decreasing the introduction of fluidizing air, and the air flow is continuously interrupted to provide for agitation of the powder. This arrangement leads to the removal of the smaller particles in the size range during the early stages of operation leading eventually to the presence of only the coarser fraction of the material for distribution. This necessitates continuing changes in operating conditions in order to maintain uniformity.
An alternative system utilizes a vibrator for sifting powder into a receiver where the powder is picked up by an air stream. It has been found, however, that the vibration of the powder results in packing and clogging. Furthermore, the necessity for agitation limits the size of such systems. Difficulties in providing uniformity are also experienced so that constant attention to flow control is required.
Another system utilizes a long fountain-type powder hopper placed across the path which the sheets must pass on the way to the delivery pile, and this system is an improvement. Typically, the system employs a textured or engraved roller upon which powder is metered by using a steel or plastic blade in contact tangentially with the roller. This provides an even coating of powder which is carried out of the hopper into an alternating high voltage field generated by a neon static tube. The powder particles are blasted off the roller surface and then fall, sometimes assisted by a gently flowing curtain of air, to the sheets. The degree of powder application is controlled by varying the speed of the roller.
In this system, the hopper, blade, roller and tube must extend across the entire width of the press, as close to the sheets as possible, and in the correct position. The result is that filling of the hopper requires shutting down the machinery, and a man then ladles powder into the hopper. This is not easily done on some machines.
Automatic filler equipment has been proposed for feeding powder into such hoppers. This equipment is limited in application to systems where nearly straight line access is available at the end of the hopper, from outside the press frames. This sometimes results in awkward or impossible placement of the filler hopper. In any event, the system has not proven reliable enough to receive general acceptance.
Applicants' aforementioned copending application is designed to overcome the problems experienced with prior art systems. That construction provides for large capacity as well as great uniformity of operation so that the attention necessary for insuring uniform powder distribution is greatly reduced. The construction specifically comprises a system for distributing powder wherein the powder is stored in a hopper and delivered to distributing means for the application of the powder to printed surfaces or for similar purposes. Metering means in the form of a roller are interposed between the hopper and the distributor. Means are provided for maintaining a substantially constant amount of powder on the roller surface, a doctor blade preferably being utilized for this purpose.
A source of compressed air is introduced into a passage which communicates with the roller surface. As the roller introduces powder into the vicinity of the passage, the compressed air picks up the powder for movement to the distributing means. One or more secondary air inlet openings communicate with the passage for supplying air or other fluid which is maintained at a pressure, such as ambient pressure, lower than the pressure of the compressed air. The low-pressure fluid is introduced in response to negative pressure conditions which are otherwise developed within the passage, this feature providing uniformity in the flow of the air-powder mixture to the distributing nozzles.